Method of making high strength refractory filaments



May 27,` 1969 F. E. WAWNER, JR 3,446,632

METHOD oF MAKING HIGH STRENGTH REFRAGTORY FILAMENTS Filed May 4,4 1965 ATTQRNEYS United States Patent O U.S. Cl. 156-3 2 Claims ABSTRACT F THE DISCLOSURE Increasing the strength of filaments having a boroncontaining surface structure by treating the filaments with aqueous nitric acid until at least about 0.5 mil has been removed from the surface of the filaments.

This invention relates to high strength refractory filaments consisting in whole or in part of boron and to methods and apparatus for making the same. More particularly, the invention relates to methods of increasing the strength of vapor-deposited boron filaments, the term filament being used herein to denote elongated elements of various cross-sectional shapes including ribbons.

The invention is based on the discovery that the tensile strengths of boron-containing, including boroncoated filaments, may be substantially increased by subjecting the filaments to the action of nitric acid in the vapor or liquid phase.

Advantageously the boron-containing filaments are produced by passing an elongated substrate at substantially constant speed through an ambient gaseous composition containing a vaporizable compound of boron while maintaining the filamentary substrate at a temperature effective to cause deposition of boron on the substrate. Typically, the gaseous composition may consist of or comprise a thermally decomposable halide or hydride of boron or a mixture of a boron halide with hydrogen. For example, the gaseous composition may consist of a mixture of a boron halide, such as boron trichloride, and hydrogen, at least a portion of the sub-V strate in contact with the gaseous composition being maintained at a temperature in the range from about 1100 K. to somewhat above 1600 K. but below the melting points of boron and the substrate whereby a massive deposit of amorphous or polycrystalline boron is formed on the substrate. Boranes, such as BZHS, may also be used as the thermally decomposable substance and are particularly suitable for deposition of boron on substrates of lower melting points, such as aluminum. The speed of lthe substrate through the zone of contact thereof with the gaseous composition is adjusted with respect to the temperature thereof and the rate of deposition of boron to obtain a deposit of the desired thickness.

Among the substrates which can advantageously be used for the preparation of boron-containing filaments are tungsten, rhenium, tantalum, titanium, molybdenum, iron, copper, nickel, Nichrome, aluminum, magnesium, carbon and graphite. X-ray diffraction data indicate that in the deposition of boron by the methods described above the boron combines with the substrate substance to form borides. In the case of tungsten substrates, the compounds WB, or W2B5 may be formed.

The 4filaments thus formed may for some purposes be subjected to a moving zone melting operation, for example, by localized electron bombardment, to convert the substance of the filament to substantially monocrystalline condition or to otherwise improve the physical properties of the filaments.

Preferably the filaments formed by deposition in the manner described above are treated with aqueous nitric acid, at a concentration of about 40% to 50% by Weight, for example, one part by volume of red furning nitric acid to one part by volume of water, at a temperature of about C. This treatment results in increases in the tensile strengths of the filaments in the range of from about 105,000 to `about 200,000 lb./in.2.

An illustrative example of the method of the invention will be described with reference to the accompanying drawing showing diagrammatically apparatus suitable for making unrestricted lengths of boron-containing filaments treated by the method of the invention.

The illustrated apparatus comprises a preheating chamber generally designated 10 and a deposition chamber generally designated 11 and an etching bath generally designated 30. A substrate filament 12 is passed continuously at a substantially uniform rate of travel from power-driven feed spool 13 successively through the preheating chamber, the deposition chamber and the etching and rinsing baths to power-driven take-up spool 14. The preheating chamber and deposition chamber are sealed from each other and from the surrounding atmosphere by means of Y-shaped mercury seals 15a, 15b, 15a` connected by fiexible tubes 24 to separate level adjusting reservoirs 16a, 16b, 16e. The mercury seals also serve as contactors for the supply of heating current to the substrate in the preheating and deposition chambers. Tension on the substrate filament is maintained substantially constant by suspending a tensioning weight 26 in a loop of the filament.

Containers 30 and 31 for .the nitric acid etching bath and the rinsing bath are provided with glass rods 32 for guiding the filament beneath the surface of the etching bath and rinsing bath.

Ts 17a, 17b, 17e and 17d and associated T-connectors 1-8 serve as inlets and outlets for gases and vapors into and out of the preheating and deposition chambers and also for the support of adjustable guide loops 19 for the substrate filament.

Hydrogen or other purging gas may be supplied to preheating chamber 10 through tube 20 and removed through Itube 21 for the purpose of removing adsorbed, absorbed or reacted oxygen from the surface of the substrate filament as it passes through chamber 10. Any lubricants or other surficial impurities are also removed from the filament by evaporation or by reaction with the hydrogen.

Boron halide may be supplied to chamber 11 by passing a carrier gas, such as argon, or a reducing gas, such as hydrogen, through a body of liquid halide in reservoir 25, which is maintained by heat transfer means (not shown) at a suitable pre-determined temperature, and passing the gas saturated with halide into the deposition chamber 11 through inlet tube 22. Residual gases and vapors pass out of chamber 11 through tube 23 to recovery or disposal means (not shown). When the thermally decomposable compound is normally gaseous, as for example, BC13 or B2H6, the reservoir 25 is not used but the gaseout compound is introduced into the reaction chamber through separate metering means.

Glass is a suitable material of construction for the outgassing and deposition chambers and for the etching bath container and for the connections thereto and therebetween including the Ts 15 and the Ts 17. The flexible tubing 20, 21, 22, 23 and 24 can be made of polyethylene and T-connectors 18 can be of copper.

The following is a representative example of the operation of the method:

An 0.5 mil tungsten wire is passed through the apparatus at a speed of 2 feet per minute. It is heated to about 1900 F. in a current of hydrogen in the outgassing chamber. In passing through the deposition chamber (about 24 inches in length) it is maintained at about 1900 F. and is contacted With hydrogen gas saturated in vessel 25 with boron tribromide at a temperature of about 77 F. (BBr3 concentration about 8 mole percent). Under these conditions a boron lament about 3 mils in nal diameter is obtained and continuous lengths up to 1000 feet of the boron filament having breaking strengths of the order of 4 105 lb./in.2 may be made.

Passage of the filament through aqueous nitric acid of approximately 40% concentration by weight at a temperature of about 100 C. in container 30 followed by rinsing in boiling water in container 31, whereby a reduction in diameter of the filament of the order of 0.5 mil is effected raises the tensile strength of the filaments to the order of 5-5.5 105 lb./in2.

When the method Iof the invention is used to treat boron filaments in the range of l0 to l5 mils in diameter, increases in exural strength from an average of about 750,000 lb./in.2 to an average of over 2,000,000 lb./in.2 are obtained.

I claim:

1. A method of increasing the strength of metallic filaments having a diameter of 3-15 mils and having a surface structure principally containing boron which cornprises etching the laments with aqueous nitric acid at a concentration of from about to 50% by weight at a temperature of about 100 C. until the diameter of such filaments has been reduced by about 0.5 mil.

2. A method of increasing the strength of metallic filaments having a surface structure principally containing boron which comprises treating the filaments with aqueous nitric acid at a concentration of from about 40% to about by weight and at a temperature of about C. until at least about 0.5 mil has been removed from the surface of the filaments.

References Cited UNITED STATES PATENTS 2,839,367 6/1958 Stern 23--209 OTHER REFERENCES Ceramic Bulletin TP785A63, November 1963, p. 693. Journal Amer. Chem. Soc. vol. 65, p. 1924-31, Boron I by Laubengayer et al.

JACOB H. STEINBERG, Primary Examiner.

U.S. Cl. X.R. 

